The Affix plugin within Bootstrap is nice, when you put something under it those contents jump up when the user reaches the point where the Affix plugin starts. Heres a simple solution without using the affix plugin to allow a header above the navbar

To get a slide down dropdown menu with bootstrap is pretty easy, and only takes a couple lines of code. You can see an example of this by hovering over the navbar links here on this site, for non members the forum link is the only one that drops down.